1. Field of the Invention
The present general inventive concept relates to an image sensor, and more particularly, to an image sensor to stabilize a black level.
2. Description of the Related Art
An image sensor converts photo images into electric signals, and the image sensor is widely used in digital cameras, game devices, vision systems, and the like.
The image sensor may be classified as a charge coupled device (CCD) type and a complementary metal-oxide semiconductor (CMOS) type. The CCD type image sensor generally has lower noise levels and better image quality than the CMOS type image sensor, but the CCD type image sensor has disadvantages in power consumption. The CMOS type image sensor may be manufactured by common semiconductor manufacturing technologies, and the CMOS type image sensor may be easily integrated into other systems such as amplifiers or signal processing units. The CMOS type image sensor also has high processing speeds and lower power consumption than the CCD type image sensor.
The CMOS image sensor may have a three-transistor pixel structure or a four-transistor pixel structure. The CMOS image sensor may have a pixel structure where pixels share one or more transistors.
FIG. 1 is a cross-sectional view of an image sensor 100 for describing a light leakage phenomenon of an optical black region.
Referring to FIG. 1, the image sensor 100 includes a substrate 110 having pixels 111, 113 and 115. In an active region, first pixels 111 are arranged in a matrix type for providing image signals by converting incident lights into electric signals. In an optical black region where the incident light is shielded by a light shielding film 140, second pixels 115 are arranged for providing a black level or a dark level corresponding to a reference level irrelevant to the incident lights. In the optical black region adjacent to the active region, additional pixels 113, i.e., dummy pixels, may be arranged.
The black level is provided by the second pixels 115 in the optical black region to be used as a reference level reflecting environmental factors such as temperature. A difference between the image signals provided by the first pixels 111 and the black level is regarded as real image signals.
As illustrated in FIG. 1, a light 151 passes through a boundary between the active region and the optical black region to reach the second pixels 115 by reflections due to wire layers 120 and 130. Also, a light 152 may reach the second pixels 115 by total reflection at a lower surface of the substrate 110. The lights 151 and 152 may distort the black level by causing a photoelectric effect to the second pixels 115. The distortion of the black level due to the lights 151 and 152 is referred to as a light leakage phenomenon. The calculated image signals based on the dark level of the second pixels 115 affected by the light leakage phenomenon are darker than the real image signals. Also one or more black crosswise lines may be caused in a displayed image and thus a quality of the displayed image may be degraded.